1. Field of the Invention
The present invention relates to an optical recording apparatus in which a beam of light is used to record information signals on a recording medium, to reproduce the recorded signals and also to erase the recorded signals.
2. Description of the Prior Art
Optical recording apparatus are know in the art. Herein, the prior art technique for optical recording will be described in connection with the case where as the optical recording medium, a photo-magnetic disk is used.
The photo-magnetic optical recording medium is prepared by forming a magnetic film on a disk substrate. The formation of the magnetic film is carried out employing a suitable known technique such as sputtering of magnetic material. The formed magnetic film is pre-magnetized uni-directionally and vertically by applying a vertical magnetic field thereto or by another method.
The recording of information on the recording medium is carried out in the following manner:
The recording medium is rotated and a beam of light (usually a laser beam) is projected on the rotating recording medium to bring the magnetic material to the state of its magnetic transfer temperature (Curie temperature) thereby causing inversion of magnetization to form record bits spirally or in concentric circles. During the exposure of the light beam, the length of a record bit is changed in the scan direction of the recording medium (in the rotational direction of the medium) by applying a signal according to the information of the beam generator. In this manner, and through application of an appropriate magnetic field, information is recorded on the photo-magnetic disk. FIG. 1 illustrates the manner of this optical recording. In FIG. 1, reference numeral 1 designates a recording beam spot and 2 designates a record bit. The width D.sub.W of the recording beam spot measured in the direction perpendicular to the scan direction (as shown in FIG. 1) is generally in the order of 1.3 to 1.8 .mu.m. The pitch P of record bit lines measured diametrically is in the order 2-3 times larger than D.sub.W.
To reproduce the recorded information, a light beam is also projected on the recording medium at such an intensity which does not cause any inversion of magnetization of the magnetic material. During the beam exposure, the magnetized pattern is detected making use of the photo-magnetic effect thereby reading the recorded information.
When it is wished to erase the recorded information, the recording medium is rotated and a beam of light is projected on the medium in the record bit area while tracking on the record bit line. Thereby, the temperature of the exposed portion is provisionally raised up to Curie point and then the portion is cooled while applying DC bias magnetic field in the same direction as the direction of magnetization of other parts than the record bit on the recording medium. In this manner, the recorded information is erased.
According to the known method, the same beam as the recording beam is used also for erasing the recorded information. FIG. 2 illustrates the known erasing method. 1' is a recording beam spot and 2' is a record bit. The recording beam is so controlled as to run just on the record bit line. However, it has been practically impossible to make the recording beam running on the record bit line perfectly even by the highest level of control and therefore there have often occurred some offset of tracking as shown in FIG. 2. If such offset of tracking occurs, the recorded information cannot be completely erased but there remains some unerased portions 2". These unerased portions have an adverse effect on the next recording on the same recording medium. This is an important drawback common to all of known optical recording apparatus of the type in which erasing of record bits is carried out with the same beam as used for recording. It is impossible to perfectly erase the record bits according to the prior art.